Inertial Navigation
Advantages
•
instantaneous output of position and velocity
•
completely self contained
•
all weather global operation
•
very accurate azimuth and vertical vector measurement
•
error characteristics are known and can be modeled quite well
•
works well in hybrid systems

Inertial Navigation
Disadvantages
•
Position/velocity information degrade with time (1-2NM/hour).
•
Equipment is expensive ($250,000/system) - older systems had
relatively high failure rates and were expensive to maintain
•
newer systems are much more reliable but still expensive to
repair
•
Initial alignment is necessary - not much of a disadvantage for
commercial airline operations (12-20 minutes)

Inertial Navigation – Basic Principle
•
If we can measure the acceleration of a vehicle we can
•
integrate the acceleration to get velocity
•
integrate the velocity to get position
•
Then, assuming that we know the initial position and velocity we
can determine the position of the vehicle at any time t.

Inertial Navigation – The Fly in the
Ointment
•
The main problem is that the accelerometer can not tell
the difference between vehicle acceleration and gravity
•
We therefore have to find
a way of
separating the
effect of gravity and the effect of acceleration

Inertial Navigation – The Fly in the
Ointment
This problem is solved in one of two ways
1.
Keep the accelerometers horizontal so that they do not sense
the gravity vector
This is the
STABLE PLATFORM MECHANIZATION
2.
Somehow keep track of the angle between the accelrometer
axis and the gravity vector and subtract out the gravity
component
This is the
STRAPDOWN MECHANIZATION

Inertial Navigation – STABLE PLATFORM
The original
inertial navigation systems (INS) were
implemented using the STABLE PLATFORM
mechanization but all new systems use the
STRAPDOWN system
We shall consider the stable platform first because it is
the easier to understand

Inertial Navigation – STABLE PLATFORM
There are three main problems
to be solved:
1.
The accelerator platform has to be mechanically isolated from
the rotation of the aircraft
2.
The aircraft travels over a spherical surface and thus the
direction of the gravity vector changes with position
3.
The earth rotates on its axis and thus the direction of the
gravity vector changes with time

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- Summer '18